Polyvinyl chloride (PVC) has enjoyed a great deal of commercial success as a material which can form useful articles. The ease at which it can be molded, extruded, calendered or formed and its low cost has contributed to this success. However, these vinyl chloride polymers are particularly susceptible to degradation by heat because of the high temperatures at which these polymers must be processed to make useful articles. To protect these vinyl chloride polymers from heat degradation, it is customary to add materials known as stabilizers.
PVC by itself is not heat stable. It would be impossible to process it in industrial machines without using stabilizers. During processing at high temperatures (about 200.degree. C.), this instability leads to the loss of hydrochloric acid and the formation of color. These two phenomena can be explained by the following reactions: ##STR1## hydrochloric acid is released from a heat sensitive site which is at the allylic chlorine. As hydrochloric acid is released, a double bond and a new active site is formed. Degradation continues, step by step releasing increasing amounts of hydrochloric acid and creating long sequences of colored interacting double bonds. This dehydrochlorination reaction is catalyzed by the presence of bases, acids and certain salts.
Hydrochloric acid and zinc chloride are particularly bad and their presence should be limited.
The heat stabilizers most commonly used to prevent this degradation are metallic salts, such as organic salts of tin, calcium and zinc.
Many times colorants are added to polyvinyl chloride to make colored articles, but in certain applications it is desirable to have an article which is transparent. Articles such as bottles, packages and films for certain markets demand a high degree of clarity. To achieve a high clarity it is customary to mix a tin stabilizer with the vinyl polymer. Unfortunately, tin stabilizers are known to impart an undesirable taste and odor to the contents of a tin stabilized PVC article. For example, if a PVC water bottle was stabilized, then within a very short time the water would have an undesirable smell and taste.
For products where taste and odor are important, it is customary to use a calcium/zinc stabilizer system. The calcium/zinc stabilizer works by neutralizing the hydrochloric acid that is formed and by stopping the propagation reaction. The zinc compound reacts with HCl to form zinc chloride. Zinc chloride is also a catalyst for degradation. This is why the calcium stabilizer is used. The calcium reacts with the zinc chloride to form calcium chloride which has no catalytic effect on degradation. The zinc salts limit the early development of color by reacting with the hydrochloric acid but cause sudden decomposition. The calcium salts delay decompsition but allow early color to develop. These opposing affect must be compromised. The calcium/zinc stabilizer system does not impart objectionable taste and odor. However, there are other problems with the calcium/zinc stabilizer system, one of which is the clarity of the article, such as a bottle, is not as clear as with the tin stabilizer. Also, the prior known calcium/zinc stabilizer system develops "blush" which is a cloudiness in the bottle wall due to exposure to alcohol and water.
The calcium/zinc stabilizer system has been approved by the U.S. Food and Drug Administration for use in contact with food and beverages. It is customary to use other materials known as co-stabilizers with the calcium/zinc stabilizer system. The most widely used calcium/zinc stabilization systems for PVC are combinations of at least four ingredients: zinc soap, calcium soap, epoxidized fatty ester, and non-toxic organic phosphite ester. The above stabilization system gives short color stability time with good yellowness or initial color. It is then necessary to use additional co-stabilizers to increase heat stability while maintaining low yellowness and good processability of the bottle compositions. Commonly, alcohols have been added which improve thermal stability but they cause "blush". The problem of "blush" is of particular importance when the bottle contains an alcohol solution such as is the case with mouthwash bottles.
The art of PVC compounding is extremely complex in that many ingredients interact with each other. This complexity means that levels of ingredients and types of ingredients must be varied to give a product which is useful for a given application. It is seldom as simple as adding one ingredient which improves a particular property but does not effect other important properties.
It would be very desirable to have a calcium/zinc stabilized PVC article, such as a bottle, which would have high clarity and low blush and be substantially or completely free of tin stabilizers so as not to impart taste and odor to the contents of the article.
It would also be desirable to improve the calcium/zinc stabilization system so as to make the system give more heat degradation resistance.